1. Field of the Invention
The present invention relates to an electroluminescence display (hereafter, referred to as an EL display). More particularly, the present invention relates to an electroluminescence display including a drive circuit that drives EL pixels at a high speed.
2. Description of the Related Art
An EL display has been widely used. FIG. 1 shows the configuration of a matrix type organic EL display. A driving circuit 101 is connected to organic EL pixels 102. The organic EL pixel 102 is connected to a horizontal drive switch 103. The horizontal drive switch 103 is connected to a ground terminal 104 and a power supply 105.
The driving circuit 101 drives one of the organic EL pixels 102 connected thereto. Which one of the organic EL pixels 102 is driven is determined by the horizontal drive switch 103. The organic EL pixel 102 is connected to any one of the ground terminal 104 and the power supply 105 by the horizontal drive switch 103, and a drive current flows through the organic EL pixel 102 connected to the ground terminal 104. That is, the organic EL pixel 102 connected to the ground terminal 104 is driven by the driving circuit 101.
On the other hand, the drive current does not flow through the organic EL pixel 102 connected to the power supply 105.
FIG. 2 shows the structure of each organic EL pixel 102. An anode 109., an organic film 110 and a cathode 111 are formed in turn on a transparent substrate 108. Electro-luminescence phenomenon causes the organic film 110 to emit a light.
FIG. 3 shows the equivalent circuit of the organic EL pixel 102. The organic EL pixel 102 is represented by the circuit in which a parasitic capacitor 112 and a light emitting diode 113 are connected parallel to each other. The parasitic capacitor 112 indicates a capacitance formed between the anode 109 and the cathode 111. A thickness of the organic film 110 is thin, typically ranging from 100 nm to 200 nm. The parasitic capacitor 112 typically has a capacitance of about 3 to 4 pF when a pixel size is 0.03 square millimeters.
FIG. 4 shows the dependency between a light emission intensity of the organic EL pixel 102 and a voltage applied to the organic EL pixel 102. The organic EL pixel 102 emits light when the voltage applied thereto exceeds a light emission start voltage VT. The light emission start voltage VT depends on color of the light, ranging from 5 to 10 V. It is necessary to charge the parasitic capacitor 112 of the organic EL pixel 102 to the light emission start voltage VT in order that the organic EL pixel 102 emits the light. A rapid charge of the parasitic capacitor 112 shortens the time necessary for the light emission of the organic EL pixel 102.
A light emitting display is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 11-231834), in which a parasitic capacitor of an EL pixel is charged at a high speed. In the conventional light emitting display, the time necessary for the light emission of the EL element is shortened by the following operation. When a drive is started, a constant charge voltage is firstly applied to the EL pixel to charge the parasitic capacitor. The charge voltage is selected such that the parasitic capacitor is charged at the high speed. In succession, a drive current to enable the light emission of a desirable intensity flows through the EL pixel. The time necessary for the light emission of the EL element is shortened by charging the parasitic capacitor at the high speed.
However, it is difficult that the conventional light emitting display has a high contrast. In order that the EL pixel emits a light at a high intensity, it is necessary to increase a charge voltage applied when the drive is started. However, the increase in the charge voltage disables the EL pixel to emit the light at a low intensity, because at least the charge voltage is applied to the EL pixel. On the other hand, if the charge voltage is decreased such that the EL pixel can emit the light at the low intensity, the EL pixel can not emit the light at the high intensity.
It is desirable that the EL display has a high contrast.
Also, the conventional light emitting display is susceptible to the influence from an ambient temperature. As shown in FIG. 5, an intensityxe2x80x94drive voltage property of an EL pixel is largely varied depending on the ambient temperature. The light emission intensity of the EL pixel largely depends on the ambient temperature, because the constant charge voltage is applied to the EL pixel light emitting display when the drive is started.
Moreover, the variation in the ambient temperature causes the tonality to be changed. This is because the variation degree of the intensityxe2x80x94drive voltage property of the EL pixel with respect to the ambient temperature is different depending on the light emission color of the EL pixel.
It is desirable that the EL display is not susceptible to the influence from the ambient temperature. In particular, it is desirable that the light emission intensity and the tonality are not susceptible to the influence from the ambient temperature.
Other techniques for driving EL pixels are disclosed in Japanese Open Laid Patent Application (JP-A-Heisei 11-45071, and JP-A-Heisei 11-282419). However, these techniques do not solve the above-mentioned problems.
Therefore, an object of the present invention is to increase a contrast of an EL display.
Another object of the present invention is to provide an EL display in which a time necessary for a light emission is shorten and a contrast is high.
Still another object of the present invention is to provide an EL display that is not susceptible to an influence from an ambient temperature.
Still another object of the present invention is to provide an EL display in which a time necessary for a light emission is shortened and it is not susceptible to an influence from an ambient temperature.
In order to achieve an aspect of the present invention, an electroluminescence display is composed of an electroluminescence pixel and a driving circuit. The driving circuit drives the electroluminescence pixel to emit light. The driving circuit provides a first drive current, and then provides a second drive current for the electroluminescence pixel. The first drive current is larger than the second drive current, and increases depending on the second drive current.
The second drive current is preferably determined based on a brightness of the light.
Also, the first drive current is preferably smaller than a limit current for maintaining a current-brightness property of the electroluminescence pixel substantially linear.
Preferably, the first drive current is k times as large as the second drive current, where k is a constant larger than 1.
The k is preferably defined such that
kxe2x89xa6Imax/Iout2-max,
where Imax is a limit current for maintaining a current-brightness property of the electroluminescence pixel substantially linear, and Iout2-max is a maximum value of the second drive current.
The k is preferably determined based on a color of light emitted by the electroluminescence pixel.
The driving circuit preferably includes a first current source unit generating a first current, a second current source unit generating a second current, and a current output unit superposing the first and second current to generate the first drive current.
The current output unit preferably generates the second drive current from the first current.
In order to achieve another aspect of the present invention, a method of operating a electroluminescence display is composed of:
providing a first drive current with a electroluminescence pixel; and
providing a second drive current with the electroluminescence pixel after the providing the first drive current. The first drive current is larger than the second drive current, and increases depending on the second drive current.